Browsing by Author "Kotilahti, Janne"

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Composite two-particle sources
(2020-02-11) Moskalets, Michael; Kotilahti, Janne; Burset, Pablo; Flindt, Christian
A2 Katsausartikkeli tieteellisessä aikakauslehdessä
Multi-particle sources constitute an interesting new paradigm following the recent development of on-demand single-electron sources. Versatile devices can be designed using several single-electron sources, possibly of different types, coupled to the same quantum circuit. However, if combined non-locally to avoid cross-talk, the resulting architecture becomes very sensitive to electronic decoherence. To circumvent this problem, we here analyse two-particle sources that operate with several single-electron (or hole) emitters attached in series to the same electronic waveguide. Using Floquet scattering theory we demonstrate how such a device can emit exactly two electrons without exciting unwanted electron-hole pairs if the driving is adiabatic. Going beyond the adiabatic regime, perfect two-electron emission can be achieved by driving two quantum dot levels across the Fermi level of the external reservoir. If a single-electron source is combined with a source of holes, the emitted particles can annihilate each other in a process which is governed by the overlap of their wave functions. Importantly, the degree of annihilation can be controlled by tuning the emission times, and the overlap can be determined by measuring the shot noise after a beam splitter. In contrast to a Hong-Ou-Mandel experiment, the wave functions overlap close to the emitters and not after propagating to the beam splitter, making the shot noise reduction less susceptible to electronic decoherence.
Computational study of the Kondo effect in a molecule/graphene system
(2017-04-25) Kotilahti, Janne
Perustieteiden korkeakoulu | Bachelor's thesis
Long-Distance Transmon Coupler with cz -Gate Fidelity above 99.8 %
(2023-01) Marxer, Fabian; Vepsäläinen, Antti; Jolin, Shan W.; Tuorila, Jani; Landra, Alessandro; Ockeloen-Korppi, Caspar; Liu, Wei; Ahonen, Olli; Auer, Adrian; Belzane, Lucien; Bergholm, Ville; Chan, Chun Fai; Chan, Kok Wai; Hiltunen, Tuukka; Hotari, Juho; Hyyppä, Eric; Ikonen, Joni; Janzso, David; Koistinen, Miikka; Kotilahti, Janne; Li, Tianyi; Luus, Jyrgen; Papic, Miha; Partanen, Matti; Räbinä, Jukka; Rosti, Jari; Savytskyi, Mykhailo; Seppälä, Marko; Sevriuk, Vasilii; Takala, Eelis; Tarasinski, Brian; Thapa, Manish J.; Tosto, Francesca; Vorobeva, Natalia; Yu, Liuqi; Tan, Kuan Yen; Hassel, Juha; Möttönen, Mikko; Heinsoo, Johannes
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Tunable coupling of superconducting qubits has been widely studied due to its importance for isolated gate operations in scalable quantum processor architectures. Here, we demonstrate a tunable qubit-qubit coupler based on a floating transmon device, which allows us to place qubits at least 2 mm apart from each other while maintaining over 50-MHz coupling between the coupler and the qubits. In the introduced tunable-coupler design, both the qubit-qubit and the qubit-coupler couplings are mediated by two waveguides instead of relying on direct capacitive couplings between the components, reducing the impact of the qubit-qubit distance on the couplings. This leaves space for each qubit to have an individual readout resonator and a Purcell filter, which is needed for fast high-fidelity readout. In addition, simulations show that the large qubit-qubit distance significantly lowers unwanted non-nearest-neighbor coupling and allows multiple control lines to cross over the structure with minimal crosstalk. Using the proposed flexible and scalable architecture, we demonstrate a controlled-Z gate with (99.81±0.02)% fidelity.
Multi-particle interference in an electronic Mach-Zehnder interferometer
(2021-06) Kotilahti, Janne; Burset, Pablo; Moskalets, Michael; Flindt, Christian
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The development of dynamic single-electron sources has made it possible to observe and manipulate the quantum properties of individual charge carriers in mesoscopic circuits. Here, we investigate multi-particle effects in an electronic Mach-Zehnder interferometer driven by a series of voltage pulses. To this end, we employ a Floquet scattering formalism to evaluate the interference current and the visibility in the outputs of the interferometer. An injected multi-particle state can be described by its first-order correlation function, which we decompose into a sum of elementary correlation functions that each represent a single particle. Each particle in the pulse contributes independently to the interference current, while the visibility (given by the maximal interference current) exhibits a Fraunhofer-like diffraction pattern caused by the multi-particle interference between different particles in the pulse. For a sequence of multi-particle pulses, the visibility resembles the diffraction pattern from a grid, with the role of the grid and the spacing between the slits being played by the pulses and the time delay between them. Our findings may be observed in future experiments by injecting multi-particle pulses into a Mach-Zehnder interferometer.
Quantum interference in dynamically driven mesoscopic conductors
(2021-08-24) Kotilahti, Janne
Perustieteiden korkeakoulu | Master's thesis
Mesoscopic conductors have become an important platform for experiments on the physics of individual electrons. They have enabled the development of electron quantum optics, where single-electron excitations are manipulated analogously to photons in quantum optics. This has led to the creation of electronic counterparts of various photonic interferometers and the possibility of using electrons as flying qubits. These advances pave the way towards future quantum technologies where single charges are controlled in nanoscale, quantum-coherent circuits to perform quantum computations. In this thesis, we study a system consisting of an electronic Mach-Zehnder interferometer and a metallic contact to which time-dependent voltage is applied. The voltage pulses excite single-electron quasiparticles called levitons, which then travel along edge states through the interferometer. By measuring the electrons ending up in different paths after the interferometer, one can find how quantum interference manifests itself in these systems. Our goal in this thesis is to improve the understanding of such experiments. To achieve this, we perform calculations based on Floquet scattering theory to analyze how quantum interference affects quantities such as the time-dependent current, transferred charge, and charge visibility. We also examine the first order correlation functions of the electrons to gain deeper analytical insights. We find that the transferred charge and current consist of cleanly separated classical and quantum contributions. The quantum parts of these quantities feature oscillations originating from quantum interference effects and can be accessed using measurements at different temperatures. Our analysis reveals that the number of these oscillations is proportional to the number of elementary charges excited by the voltage. We also show how the zeros of the correlation function encode all the relevant information about the interference of the excited particles. The results of this thesis form a theoretical guide for future experiments on interferometry of single- or few-electron excitations.
Time-Domain Spectroscopy of Mesoscopic Conductors Using Voltage Pulses
(2019-02-27) Burset, Pablo; Kotilahti, Janne; Moskalets, Michael; Flindt, Christian
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The development of single-electron sources is paving the way for a novel type of experiment in which individual electrons are emitted into a quantum-coherent circuit. However, to facilitate further progress toward fully coherent on-chip experiments with electrons, a detailed understanding of the quantum circuits is needed. Here, it is proposed to perform time-domain spectroscopy of mesoscopic conductors by applying Lorentzian-shaped voltage pulses to an input contact. Specifically, it is shown how characteristic timescales of a quantum-coherent conductor can be extracted from the distribution of waiting times between charge pulses propagating through the circuit. To illustrate the idea, Floquet scattering theory is employed to evaluate the electron waiting times for an electronic Fabry–Pérot cavity and a Mach–Zehnder interferometer. The perspectives for an experimental realization of the proposal are discussed and possible avenues for further developments are identified.
Unimon qubit
(2022-11-12) Hyyppä, Eric; Kundu, Suman; Chan, Chun Fai; Gunyhó, András; Hotari, Juho; Janzso, David; Juliusson, Kristinn; Kiuru, Olavi; Kotilahti, Janne; Landra, Alessandro; Liu, Wei; Marxer, Fabian; Mäkinen, Akseli; Orgiazzi, Jean Luc; Palma, Mario; Savytskyi, Mykhailo; Tosto, Francesca; Tuorila, Jani; Vadimov, Vasilii; Li, Tianyi; Ockeloen-Korppi, Caspar; Heinsoo, Johannes; Tan, Kuan Yen; Hassel, Juha; Möttönen, Mikko
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Superconducting qubits seem promising for useful quantum computers, but the currently wide-spread qubit designs and techniques do not yet provide high enough performance. Here, we introduce a superconducting-qubit type, the unimon, which combines the desired properties of increased anharmonicity, full insensitivity to dc charge noise, reduced sensitivity to flux noise, and a simple structure consisting only of a single Josephson junction in a resonator. In agreement with our quantum models, we measure the qubit frequency, ω01/(2π), and increased anharmonicity α/(2π) at the optimal operation point, yielding, for example, 99.9% and 99.8% fidelity for 13 ns single-qubit gates on two qubits with (ω01, α) = (4.49 GHz, 434 MHz) × 2π and (3.55 GHz, 744 MHz) × 2π, respectively. The energy relaxation seems to be dominated by dielectric losses. Thus, improvements of the design, materials, and gate time may promote the unimon to break the 99.99% fidelity target for efficient quantum error correction and possible useful quantum advantage with noisy systems.